Enhanced photodetector

ABSTRACT

The present invention includes a photodiode having a first p-type semiconductor layer and an n-type semi-conductor layer coupled by a second p-type semiconductor layer. The second p-type semiconductor layer has graded doping along the path of the carriers. In particular, the doping is concentration graded from a high value near the anode to a lower p concentration towards the cathode. By grading the doping in this way, an increase in absorption is achieved, improving the responsivity of the device. Although this doping increases the capacitance relative to an intrinsic semiconductor of the same thickness, the pseudo electric field that is created by the graded doping gives the electrons a very high velocity which more than compensates for this increased capacitance.

FIELD OF THE INVENTION

The present invention relates to a semiconductor-based photodetector,and in particular to a high-speed, broad bandwidth photodetector havingenhanced absorption characteristics.

BACKGROUND AND SUMMARY OF THE INVENTION

There is a well-known tradeoff between high speed and sensitivity in aphotodetector. High bandwidth signal detection requires a short transittime of the carriers and thus a thin absorption layer. However, thegeometrical constraints on the absorption layer thickness results in areduced absorption and lower responsivity.

One type of semiconductor-based photodetector is termed a p-i-n junctiondiode, or a PIN diode. This type of structure is generally composed of anumber of solid semiconductive sandwiched together in an epitaxialstructure. In particular, a p-type semiconductor material and an n-typesemiconductor region are separated by an intrinsic semiconductor.

In a PIN diode, the depletion layer extends into each side of junctionby a distance that is inversely proportional to the dopingconcentration. Thus, the p-i depletion layer extends well into theintrinsic material, as does the depletion layer of the i-n junction.Accordingly, a PIN diode functions like a p-n junction with a depletionlayer that encompasses the entirety of the intrinsic material. Theprimary advantages inherent to this structure are twofold. First, theaddition of the intrinsic layer permits a fractional increase in theamount of light to be captured by the diode. Secondly, due to theextended depletion layer, the PIN diode has a very small junctioncapacitance and corresponding fast response.

Most attempts at increasing the speed of PIN diodes have focused onreducing the capacitance at the junction. At least one proposed designhas included an undoped drift region for this purpose, effectivelyincreasing the size of the intrinsic portion of the diode. Although thissolution is suitable for decreasing the junction capacitance, itunfortunately increases the transit time for the carriers and thusreduces the response time of the photodetector. As such, there is a needin the art for an improved photodetector that strikes the proper balancebetween capacitance and response time, while increasing the responsivityof the device.

Accordingly, the present invention includes a photodiode having a firstp-type semiconductor layer and an n-type semiconductor layer coupled bya second p-type semiconductor layer. The second p-type semiconductorlayer has graded doping along the path of the carriers. In particular,the doping is concentration graded from a high value near the anode to alower p concentration towards the cathode. By grading the doping in thisway, an increase in absorption is achieved, improving the responsivityof the device. Although this doping increases the capacitance relativeto an intrinsic semiconductor of the same thickness, the pseudo electricfield that is created by the graded doping gives the electrons a veryhigh velocity which more than compensates for this increasedcapacitance. Further embodiments and advantages of the present inventionare discussed below with reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy band diagram of a pin photodiode in accordance withthe present invention.

FIG. 2 is a cross-sectional view of a basic configuration of a pinphotodiode in a surface illuminated structure in accordance with thepresent invention.

FIG. 3 is a graph representing the relationship between the electricfield and the electron velocity according to an aspect of the presentinvention.

FIG. 4 is a graph representing the relationship between the dopingconcentration and the relative depth of a semiconductor layer of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with a preferred embodiment of the present invention, anepitaxial structure is provided for photoconductive purposes. Thephotoconductive structure is a modified PIN diode that is optimized forincreased performance through an enhanced layer having a graded dopingconcentration. The particulars of the structure and method ofmanufacture of the present invention are discussed further herein.

Referring to FIG. 1, an energy band diagram of a PIN photodiode 10 showsthe relative energy levels of the semiconductor materials that form thephotodiode 10. In particular, the photodiode 10 is comprised of a groupof semiconductor materials, including a first p-type semiconductor layer14, a second p-type semiconductor layer 16, and an n-type semiconductorlayer 18. An anode layer 12 is shown adjacent to the first p-typesemiconductor layer 14 to collect holes.

The first p-type semiconductor layer 14 is selected from a groupcomprising tertiary semiconductors, or group III-V semiconductors.Accordingly, the first p-type semiconductor layer 14 is either twoelements from group III combined with one element from group V or theconverse, two elements from group V combined with one element from groupII. A table of representative groups of the periodic table is shownbelow. GROUP II GROUP III GROUP IV GROUP V Zinc (Zn) Aluminum (Al)Silicon (Si) Phosphorus (P) Cadmium (Cd) Gallium (Ga) Germanium (Ge)Arsenic (As) Mercury (Hg) Indium (In) Antimony (Sb)

In the preferred embodiment, the first p-type semiconductor layer 14 isInAlAs. However, it is understood that the first p-type semiconductorlayer 14 may be any tertiary semiconductor that provides the necessarybandgap for optimized operation of the photodiode 10.

The n-type semiconductor layer 18 is also selected from a groupcomprising tertiary semiconductors, or group II-V semiconductors. Asbefore, the n-type semiconductor layer 18 is either two elements fromgroup III combined with one element from group V or the converse, twoelements from group V combined with one element from group II. In thepreferred embodiment, the n-type semiconductor layer 18 is InAlAs.However, it is understood that the n-type semiconductor layer 18 may beany tertiary semiconductor that provides the necessary bandgap foroptimized operation of the photodiode 10.

The second p-type semiconductor layer 16 is also selected from a groupcomprising tertiary semiconductors, or group III-V semiconductors. Inthe preferred embodiment, the second p-type semiconductor layer 16 isInGaAs with a graded doping concentration. However, it is understoodthat the second p-type semiconductor layer 16 may be any tertiarysemiconductor that provides the necessary low bandgap for optimizedoperation of the photodiode 10.

In order to achieve a graded doping concentration, the second p-typesemiconductor layer 16 is not doped in a typical manner. In general, ap-type semiconductor is fabricated by using dopants with a deficiency ofvalence electrons, also known as acceptors. The p-type doping results inan abundance of holes. For example, in a type III-V semiconductor, someof the group III atoms may be replaced with atoms from group II, such asZn or Cd, thereby producing a p-type material. Similarly, as group IVatoms act as acceptors for group V atoms and donors for group III atoms,a group IV doped III-V semiconductor will have an excess of bothelectrons and holes.

FIG. 2 is a cross-sectional view of a basic configuration of aphotodiode 10 in a surface illuminated structure designed in accordancewith the present invention. A substrate layer 20 is provided for growingthe semiconductor structure. The n-type semiconductor layer 18 isdeposited upon the substrate. The first p-type semiconductor layer 14and the second p-type semiconductor layer 16 are deposited in a mannersuch that the second p-type semiconductor layer 16 is directly adjacentto the n-type semiconductor layer 18. As before, an anode layer 12 isdeposited on the first p-type semiconductor layer 14 for collectingholes. Also shown is a cathode layer 22, or n-type contact layer, forcollecting electrons.

As noted, it is a feature of the second p-type semiconductor layer 16that it includes a graded doping concentration. The presence of dopantsin the second p-type semiconductor layer 16 is controlled in order tooptimize the performance of the photodiode. A first concentration 15 islocated near the first p-type semiconductor 14, and a secondconcentration 17 is directly adjacent to the n-type semiconductor 18.Preferably, the first concentration 15 is between 800 and 1,000angstroms deep, i.e. the dimension parallel to the travel of thecarriers.

In the preferred embodiment, the first concentration 15 is greater thanthe second concentration 17. In particular, the first concentration 15is located at a position x_(c) and defines a dopant concentration p_(o).A preferred doping concentration gradient is governed by the followingequation: $\begin{matrix}{p = {p_{o}{\mathbb{e}}^{\frac{- x}{D}}}} & (1)\end{matrix}$over the depth D of the second p-type semiconductor layer 16 for all xand D greater than zero. A graph representative of Equation (1) is shownin FIG. 4.

The graded doping structure of the second p-type semiconductor layer 16results in improved performance of the photodiode 10. During operation,incident light is absorbed in the second p-type semiconductor layer 16of the photodiode 10. The light that is absorbed in the secondconcentration 17 part of the second p-type semiconductor layer 16produces electrons and holes which drift to the anode 12 and cathode 22under the influence of the large drift electric field. Although this isthe usual situation in standard uniformly low doped absorber PINphotodetectors, in the present invention, the photoresponse of thecarriers is more complex.

The electrons generated in the second concentration 17 part of thesecond p-type semiconductor layer 16 reach the cathode with theirsaturation velocity and are collected. The holes generated in the secondconcentration 17 part of the second p-type semiconductor layer 16 travelto the anode 12, thus entering the first concentration 15 where theconcentration of dopants is relatively high and where they arecollected, thus ending their transit time.

By way of comparison, the light that is absorbed in the firstconcentration 15 part of the second p-type semiconductor layer 16 alsoproduces electrons and holes. In this case however, the holes arereadily collected in the first concentration 15 and thus do not addsubstantially to the transit time of the carriers or reduce thebandwidth of the photodiode 10. Accordingly, insofar as the holes areconcerned, the graded doping concentration of the photodiode 10 does notadd to their transit time or reduce the detector bandwidth in either inthe first concentration 15 or the second concentration 17.

Another aspect of the graded doping concentration of the second p-typesemiconductor layer 16 is the creation of a pseudo-electric field. Theelectrons generated in the first concentration 15 region are subject tothis pseudo-field shown below as $\begin{matrix}{{E = {{- \left( \frac{kT}{q} \right)}\frac{\mathbb{d}p}{\mathbb{d}x}}},} & (2)\end{matrix}$where k is Boltzman's constant, T is the temperature, q is the charge ofan electron, and the value $\frac{\mathbb{d}p}{\mathbb{d}x}$is the doping concentration gradient.

The pseudo-field E produces an “overshoot” electron velocity, i.e. theelectron velocity is potentially many times faster than the saturationvelocity. A typical electron saturation velocity is on the order of5×10⁶ cm/sec. However, the exponential gradient shown in Equation (1)with D=1,000 angstroms yields a field E=2.5 kV/cm, which corresponds toan electron overshoot velocity as large as 3×10⁷ cm/sec. A graphdepicting the relationship between the magnitude of the pseudo-field Eand the electron velocity is shown in FIG. 3.

As described, the present invention improves upon the state of the artin photodiodes by implementing a graded doping concentration. In such amanner, the net absorption of a photodiode can be increased withoutsubstantially reducing the overall bandwidth of the device. It isfurther understood that it may be advantageous to optimize the overallspeed by adjusting the doping concentration, the capacitance of thedevice, and the total thickness of the absorption region. It should beapparent to those skilled in the art that the above-describedembodiments are merely illustrative of but a few of the many possiblespecific embodiments of the present invention. Numerous and variousother arrangements can be readily devised by those skilled in the artwithout departing from the spirit and scope of the invention as definedin the following claims.

1. A photodiode comprising: a first p-type semiconductor layer; ann-type semiconductor layer; a second p-type semiconductor layer disposedbetween the first p-type semiconductor layer and the n-typesemiconductor layer such that the second p-type semiconductor isdirectly adjacent to the n-type semiconductor, the second p-typesemiconductor layer having a graded doping concentration.
 2. Thephotodiode of claim 1 further comprising an anode layer for collectingholes.
 3. The photodiode of claim 1 further comprising a cathode layerfor collecting electrons.
 4. The photodiode of claim 1 wherein the firstp-type semiconductor layer is InAlAs.
 5. The photodiode of claim 1wherein the n-type semiconductor layer is InAlAs.
 6. The photodiode ofclaim 1 wherein the second p-type semiconductor layer is InGaAs.
 7. Thephotodiode of claim 1 wherein the graded doping concentration defines afirst concentration adjacent to the first p-type semiconductor layer anda second concentration adjacent to the n-type semiconductor layer, andfurther wherein the first concentration is greater than the secondconcentration.
 8. The photodiode of claim 7 wherein the firstconcentration is ocated at a position x_(o) and defines a concentrationp_(o), and further wherein the graded doping concentration is governedby the following equation: $p = {p_{o}{\mathbb{e}}^{\frac{- x}{D}}}$over the depth D of the second p-type semiconductor layer for all x andD greater than zero.
 9. The photodiode of claim 8 wherein the depth D isbetween 800 and 1000 angstroms in length.
 10. A method of fabricating aphotodiode comprising the steps of: providing a substrate layer;depositing a first p-type semiconductor layer on the substrate;depositing an n-type semiconductor layer on the substrate; grading asecond p-type semiconductor layer from a first concentration to a secondconcentration, wherein the first concentration is greater than thesecond concentration; and depositing the second p-type semiconductorlayer between the first p-type semiconductor layer and the n-typesemiconductor layer such that the second concentration is directlyadjacent to the n-type semiconductor layer.
 11. The method of claim 10further comprising the step of affixing an anode to collect holes. 12.The method of claim 10 further comprising the step of affixing a cathodeto collect electrons.
 13. The method of claim 10 wherein the firstp-type semiconductor layer is InAlAs.
 14. The method of claim 10 whereinthe n-type semiconductor layer is InAlAs.
 15. The method of claim 10wherein the second p-type semiconductor layer is InGaAs.
 16. The methodof claim 10 wherein the first concentration is located at a positionx_(o) and defines a concentration p_(o), and further wherein the gradeddoping concentration is governed by the following equation:$p = {p_{o}{\mathbb{e}}^{\frac{- x}{D}}}$ over the depth D of the secondp-type semiconductor layer for all x and D greater than zero.
 17. Aphotodiode having a first p-type semiconductor layer and an n-typesemiconductor layer comprising: a second p-type semiconductor layerdisposed between the first p-type semiconductor layer and the n-typesemiconductor layer such that the second p-type semiconductor isdirectly adjacent to the n-type semiconductor, the second p-typesemiconductor layer having a graded doping concentration, wherein thegraded doping concentration is governed by the following equation:$p = {p_{o}{\mathbb{e}}^{\frac{- x}{D}}}$ over the depth D of the secondp-type semiconductor layer for all x and D greater than zero.
 18. Thephotodiode of claim 17 wherein the second p-type semiconductor layer isa type III-V semiconductor.
 19. The photodiode of claim 17 wherein thesecond p-type semiconductor layer is InGaAs.